Field of the Disclosure
The present disclosure relates to a method of removing or substantially reducing stick-slip oscillations in a drillstring, to a method of drilling a borehole, to drilling mechanisms for use in drilling a borehole, and to an electronic controller for use with a drilling mechanism.
Background to the Disclosure
Drilling an oil and/or gas well involves creation of a borehole of considerable length, often up to several kilometers vertically and/or horizontally by the time production begins. A drillstring comprises a drill bit at its lower end and lengths of drill pipe that are screwed together. The whole drillstring is turned by a drilling mechanism at the surface, which in turn rotates the bit to extend the borehole. The rotational part of the drilling mechanism is typically a topdrive consisting of one or two motors with a reduction gear rotating the top drillstring with sufficient torque and speed. A machine for axial control of the drilling mechanism is typically a winch (commonly called drawworks) controlling a travelling block, which is connected to and controls the vertical motion of the topdrive.
The drillstring is an extremely slender structure relative to the length of the borehole, and during drilling the drillstring is twisted several turns due to the total torque needed to rotate the drillstring and the bit. The torque may typically be on the order of 10-50 kNm. The drillstring also displays a complicated dynamic behavior comprising axial, lateral and torsional vibrations. Simultaneous measurements of drilling rotation at the surface and at the bit have revealed that the drillstring often behaves as a torsional pendulum, i.e. the top of the drillstring rotates with a constant angular velocity, whereas the drill bit performs a rotation with varying angular velocity comprising a constant part and a superimposed torsional vibration. In extreme cases, the torsional part becomes so large that the bit periodically comes to a complete standstill, during which the drillstring is torqued-up until the bit suddenly rotates again and speeds up to an angular velocity exceeding the topdrive speed. This phenomenon is known as stick-slip, or more precisely, torsional stick-slip. Measurements and simulations have also revealed that the drillstring can sometimes exhibit axial stick-slip motion, especially when the drillstring is hoisted or lowered at a moderate speed. This motion is characterized by large axial speed variations at the lower end of the drillstring and can be observed at the surface as substantial oscillations of the top tension, commonly called the hook load. The observed stick-slip oscillation period is most often close to the period of the lowest natural resonance mode.
Torsional stick-slip has been studied for more than two decades and it is recognized as a major source of problems, such as excessive bit wear, premature tool failures and poor drilling rate. One reason for this is the high peak speeds occurring during in the slip phase. The high rotation speeds in turn lead to secondary effects like extreme axial and lateral accelerations and forces.
A large number of papers and articles have addressed the stick-slip problem. Many papers focus on detecting stick-slip motion and on controlling the oscillations by operational means, such as adding friction reducers to the mud, changing the rotation speed or the weight on bit. Even though these remedies sometimes help, they are either insufficient or they represent a high extra costs.
A few papers also recommend applying smart control of the topdrive to dampen and prevent stick-slip oscillations. In IADC/SPE 18049 it was demonstrated that torque feed-back from a dedicated drillstring torque sensor could effectively cure stick-slip oscillations by adjusting the speed in response to the measured torque variations. In Jansen, J. D et al. “Active Damping of Self-Excited Torsional Vibrations in Oil Well Drillstrings”, 1995, Journal of Sound and Vibrations, 179(4), 647-668, it was suggested that the drawback of this approach is the need for a new and direct measurement of the drillstring torque, which is not already available. U.S. Pat. No. 5,117,926 disclosed that measurement as another type of feed-back, based on the motor current (torque) and the speed. This system has been commercially available for many years under the trade mark SOFT TORQUE®. The main disadvantage of this system is that it is a cascade control system using a torque feed-back in series with the stiff speed controller. This increases the risk of instabilities at frequencies higher than the stick-slip frequency, especially if there is a significant (50 ms or more) time delay in the measurements of speed and torque.
The patent application PCT/GB2008/051144 discloses a method for damping stick-slip oscillations, the maximum damping taking place at or near a first or fundamental (i.e. lowest frequency) stick-slip oscillation mode. In developing the present method a further problem to be addressed when the drillstring is extremely long (greater than about 5 km) and the fundamental stick-slip period exceeds about 5 or 6 s has been identified. Even though the method according to this document is able to cure the fundamental stick-slip oscillation mode in such drillstrings, as soon as these oscillations are dampened, the second natural mode tends to become unstable and grow in amplitude until full stick-slip is developed at the higher frequency. In certain simulations it has been found that this second mode has a natural frequency which is approximately three times higher than the fundamental stick-slip frequency. The higher order stick-slip oscillations are characterized by short period and large amplitude cyclic variations of the drive torque. Simulations show that the bit rotation speed also in this case varies between zero and peak speeds exceeding twice the mean speed.
A more recent patent application PCT/GB2009/051618 discloses some improvements of the preceding application, such as inertia compensation term in combination with a slight detuning of the topdrive speed controller. These improvements broaden the absorption band width and enable the topdrive to effectively dampen also the second torsional mode, thus preventing second mode stick-slip from occurring. Another improvement is a method for real-time estimation of the rotational bit speed, based on the dynamic drive torque variations. Field experience and also extensive testing with an advanced simulation model have shown that all of the current systems for damping stick-slip oscillations sometimes fail to solve the stick-slip problem, and especially in very long drillstrings, say >5000 m. All active systems mentioned above have in common that they modify the speed of topdrive in response to a varying torque load. The resulting damping is sometimes but not always sufficiently strong to remove stick-slip oscillations. The systems have also proved to be very sensitive to noise and delay of the control signals i.e. speed and torque so that even a small time delay in order of 50 ms can cause instability to occur at higher frequencies.
The purpose of the disclosed embodiments is to overcome or reduce at least one of the disadvantages of the prior art.
The purpose is achieved according to various embodiments by the features as disclosed in the description below and in the following patent claims.